[build] Allow sparse to find compiler.h

sparse seems to have problems finding compiler.h when specified as
"-include compiler.h"; one possible explanation is that it ignores the
include path.  Fix by using "-include include/compiler.h".

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[build] Define __WINT_TYPE__ if necessary

sparse does not define __WCHAR_TYPE__ or __WINT_TYPE__.  We already
define __WCHAR_TYPE__ if the compiler does not do so; do the same for
__WINT_TYPE__.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[build] Allow sparse to be invoked via "make C=1"

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[build] Avoid sparse undeclared symbol warning for PROVIDE_SYMBOL()

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[process] Mark process descriptor as static in PERMANENT_PROCESS

There is no need for the process descriptor to be a global variable.

Signed-off-by: Michael Brown <mcb30@ipxe.org>